In both domestic and industrial applications, preheating of hot water by solar energy is coming into widespread use. Generally a solar collector for absorbing light and converting the light to thermal energy is mounted on a roof. Because use of the hot water is periodic, the heat must be transferred to an insulated storage medium which holds the heat energy for subsequent use.
In early solar water heating systems such as disclosed in U.S. Pat. Nos. 1,747,826 to Gould and 1,753,227 to Wheeler et al. hot water pipes in a solar collector were connected to a hot water storage tank. The heated water flowed into the tank by convection and to that end the solar collector was at a level lower than the water storage tank. Because it is generally preferred to have the solar collector at a high point such as on a roof and the water storage at a low point such as in a basement, such convection systems are not usually practical. Further, unless the solar collector is disconnected at night, cooling of the stored hot water may result. Thus some temperature responsive valve control is required. And once disconnected, if water is then allowed to remain in the collector it is subject to freezing.
A modern adaptation of the older water heating systems is found in U.S. Pat. No. 4,010,734 to Chayet. In that patent a temperature controlled pump is used to pump water from a hot water storage tank to the solar collector. A supplemental electrical heating element is also provided in the storage tank. Such a system requires temperature control circuitry as well as an electrical pump. With periodic turning on and off of the pump, the pump and circuitry are subject to failure. The system also suffers from the freezing problem if water is permitted to remain in the collector on cold nights.
Most conventional solar water heating systems avoid the freezing problem by using a nonfreezing heat transfer fluid. The transfer fluid is pumped in a closed circuit which passes in heat exchange relationship with the water to be heated. At in the Chayet system, temperature controls and a pump are required for the heat transfer fluid. The systems also suffer from the risk of contamination of the hot water with leakage of the nonfreezing heat transfer fluid. Further, such systems are expensive. Heat transfer fluid is much more expensive than plain water, and to avoid contamination of the water and to avoid other leakage of the expensive heat transfer fluid, a very tight and thus expensive fluid transfer system must be provided.
An object of the present invention is to provide a simplified solar water system which does not require a pump or temperature control circuitry and which thus avoids the expense and unreliability of those elements of the system.
A further object of this invention is to provide such a simplified solar heating system which does not present the risk of contamination of the heated water while still avoiding freezing of the water.
Yet another object of this invention is to provide a solar water heating system capable of providing a substantial amount of the heat requirements in a home, office building or the like.